Make it possible to list all topics with Consumer#metadata
Refactor the Consumer#metadata specs to use expect/have_received

Instead of expect/to/receive.

Using have_received means that the assertions are at the end of the test instead of in the test setup.
Add a test for the request type sent by Consumer#metadata

Since it’s important that the Scala code gets the right type.
Alias Consumer#metadata as #topic_metadata

This fixes #2
Merge branch 'iconara-consumer_metadata_improvements'